Stylus devices, commonly used for input by tablet computing devices, are typically implemented using one or more of ultrasound, surface acoustic waves, electro-magnetic, and electro-static systems. In electro-magnetic systems, a sensor array is typically installed behind the display of the tablet device. The position of the stylus device with respect to the display is determined based on signals received from the sensory array. Electro-static systems may use an array of transparent electrodes to sense a signal emitted from a stylus device. While these systems are effective, the sensor arrays must be built into the display and therefore cannot be added by a user at a later time.